1. Technical Field
The present invention relates generally to satellite navigation technology and, more particularly, to a scheme for tracking a composite binary offset carrier (CBOC) signal that is used for satellite navigation technology.
2. Description of the Related Art
Satellite navigation technology is technology that is configured such that, when a plurality of navigation satellites randomly transmits a plurality of satellite navigation signals, each containing information about the current location and time of the corresponding navigation satellite, to the ground, a satellite navigation receiver on the ground receives the plurality of satellite navigation signals, calculates the coordinates of the current locations of the navigation satellites and the arrival times of the signals, and determines its three-dimensional (3D) location in the Earth coordinate system using triangulation.
A satellite navigation receiver theoretically requires at least three satellite signals in order to determine its longitude, latitude and height, and requires one more satellite signal in order to improve accuracy by eliminating time error between satellites. Accordingly, at least four satellites are required.
Across the world, many countries have developed independent satellite navigation systems for economic and military reasons. Although the United States Global Positioning System (GPS) is most widely used and famous, the European Union's Galileo system, the Russian GLONASS, the Chinese COMPASS system, and the Japanese QZSS system (which will be expanded to the JRANS system in the future) are also being currently operated or developed.
Since satellite navigation signals should be robust to interference and jamming, a variety of elaborate modulation schemes have been employed. It is worthy of notice that the majority of the next-generation satellite navigation systems have replaced a conventional a phase shift keying (PSK) modulation scheme or have employed a BOC modulation scheme in addition to a PSK modulation scheme. The width of the main peak of an autocorrelation function used for the BOC modulation scheme is short, and thus the BOC modulation scheme exhibits better signal tracking performance than the PSK modulation scheme.
Furthermore, the BOC modulation scheme is characterized in that spectral separation occurs and energy is shifted from the center of a band to the periphery thereof, unlike the PSK modulation scheme, and thus the BOC modulation scheme can be additionally applied to a band in which a conventional modulation scheme has been used. Using these characteristics, the next-generation satellite navigation systems can employ the BOC modulation scheme in addition to the PSK modulation scheme, thereby being able to ensure the improvement of performance and backward compatibility.
A BOC signal is a signal that is expressed as a product of a pseudo random noise (PRN) code with a sine or cosine rectangular sub-carrier. A BOC signal is expressed as a BOCsin(kn,n) or a BOCcos(kn,n) depending on the type of sub-carrier, where k is a positive integer indicative of the ratio of the chip period of a PRN code to the period of a sub-carrier, and n is indicative of the ratio of PRN code chip transmission rate to 1.023 MHz, that is, the clock frequency of a C/A code.
Although a BOC signal has high signal tracking performance and excellent compatibility with the conventional PSK modulation scheme, it is problematic in that many side peaks occur around a main peak where an autocorrelation function has the highest value, unlike the PSK scheme having a single peak. A problem in which, upon tracking a BOC signal, synchronization is established with a side peak instead of a main peak due to the presence of side peaks, that is, the so-called ambiguity problem, may occur.
Meanwhile, in order to modernize the GPS system while maintaining its backward compatibility and provide compatibility between the GPS system and the Galileo system, a multiplexed BOC (MBOC) modulation method was proposed, and the U.S. and European authorities finally decided to adopt a so-called MBOC(6,1,1/11) modulation method in which a BOCsin(1,1) signal and a BOCsin(6,1) signal were combined at a power split ratio of 1/11 after discussion.
Interestingly, the U.S. and European authorities implemented different methods of synthesizing sub-carrier signals BOC(1,1) and BOC(6,1) that could satisfy the power spectrum density of the MBOC(6,1,1/11) modulation method. First, the U.S. authority implemented a time-multiplexed BOC (CBOC) using two sub-carriers BOC(1,1) and BOC(6,1) in the time domain in an non-overlap manner. In contrast, the European authority implemented a composite BOC (CBOC) in which a sub-carrier BOC(6,1) has been added to a sub-carrier BOC(1,1) along the time axis.
A CBOC modulation scheme is a method of simply summing a BOCsin(1,1) and BOCsin(6,1) in a weighted manner so that the power spectrum density of an MBOC(6,1,1/11) modulation scheme can be satisfied.
Meanwhile, a decision was made such that 50% of the power of a CBOC(6,1,1/11) signal was assigned to each of data and a pilot. For this purpose, the overall signal is divided into a CBOC(6,1,1/11,‘+’) signal for the transmission of a data component and a CBOC(6,1,1/11,‘−’) signal for signal synchronization using a pilot component.
The peak of the autocorrelation function of the CBOC(6,1,1/11) signal is sharper thanks to the advantage of a BOC(6,1) signal component, and thus can provide more accurate positioning performance than a general BOC modulated signal.
However, since the CBOC(6,1,1/11) signal has various side peaks around a main peak like a general BOC modulated signal, it still has the ambiguity problem upon signal tracking.
Proposed conventional schemes for eliminating the side peaks of a CBOC autocorrelation function are schemes for applying a conventional method of eliminating side peaks in a BOC signal without change or schemes for eliminating side peaks using a newly designed local signal. Although these schemes can actually eliminate side peaks, tracking performance is not improved.